HFC transmitters are commonly used in the CATV industry to send a broadband carrier multiplexed RF signal containing content such as television channels, video on demand and cable modem data from a head end or substation to a remote location. The HFC transmitter converts the broadband RF electrical signal, typically carried on a coaxial cable, into a primarily amplitude modulated optical signal that is sent over fiber optics to the destination where the signals is converted back to a broadband electrical signal using a high speed photodetector. The resulting electrical signal is then typically amplified and distributed over coaxial cables to the end users. This method of RF signal distribution has several advantages over a pure coaxial cable distribution method. Fiber optic cables have much lower loss than coaxial electrical cables, so signals can be transmitted much further before requiring amplification. Also, fiber optics are immune to RF interference, so the signal do not degrade due to RF ingress from external sources during transmission. Finally, multiple optical channels can be combined onto a single fiber, allowing multiple broadband RF signals to be sent over the same fiber. This is commonly done to segment the distribution network into smaller subscriber groups without the need to install additional cables. For these reasons and others not mentioned, a HFC distribution system is usually the most cost effective way to distribute CATV signals.
One of the more cost effective ways to make an HFC transmitter is using a directly modulated Distributed Feedback (DFB) semiconductor laser. However, DFB lasers suffer from chirp, which is unwanted optical frequency modulation that occurs in correlation with the optical amplitude modulation. Although this chirp helps increase the SBS threshold of the transmitter, enabling high optical launch powers and long transmission distances, it also causes a large amount of in-band Inteferometric Intensity Noise (IIN) and leads to chirp-dispersion distortion of the RF signal. Chirp-dispersion distortion is particularly problematic in the C-band (˜1550 nm) because most installed fiber has a large dispersion coefficient in this band. The C-band is usually the preferred band to transmit at because the optical loss of most installed fiber is the smallest in the C-band, the C-band optical channels can be readily amplified with an Erbium Doped Fiber Amplifier (EDFA) and it's possible to multiplex multiple optical channels onto a single fiber in the C-band with minimal impairments.
One method to overcome the signal degradation caused by IIN and chirp-dispersion distortion in the C-band is to reduce or eliminate the chirp or optical frequency/phase modulation that occurs in conjunction with the amplitude modulation. Low or no chirp amplitude modulation can be accomplished by externally modulating an optical carrier using a Mach-Zehnder (MZ) modulator or an Electro-Absorption (EA) modulator. Also, a directly modulated DFB laser paired with an optical phase modulator that compensates for the chirp can produce a low chirp output, see U.S. Pat. No. 7,848,661 and U.S. Pat. No. 7,936,997 the entire contents of both of which are hereby incorporated herein for all purposes. Regardless of the method used to produce a low chirp optical amplitude modulation, reducing or eliminating the chirp causes a reduction in the SBS threshold due to the reduction in optical linewidth. SBS is non-linear effect that limits launch power. When too much optical power is contained in too narrow of a band, the fiber starts to act like a Distributed Bragg Reflector and the power is reflected back to the source. This effect can severely limit the optical launch power into the fiber, which reduces the effective transmission distance.
To overcome the problem of low SBS thresholds in low chirp HFC transmitters, a high frequency optical phase/frequency modulation can be added. This phase/frequency modulation increases the effective optical linewidth of the laser, splitting the optical carrier into multiple lines with lower optical power, which increases the SBS threshold. There are several ways to modulate the optical phase/frequency of the transmitter including, but not limited to, using an optical phase modulator or directly modulating the drive laser of an externally modulated transmitter.
An optical phase modulator is a waveguide device made out a material whose index of refraction changes with applied electrical field. By applying a high frequency electrical modulation, a high frequency optical phase modulation can be produced. Directly modulating the drive lasers of an externally modulated transmitter can produce a large optical frequency modulation due to the large chirp parameter of these lasers. Regardless of the method of obtaining the optical phase/frequency modulation, the frequency can be greater than 2 times the highest transmission signal frequency in order to avoid signal degradation due to intermodulation effects. For example, if it is desired to transmit a 1 GHz broadband RF signal, the optical phase modulation can be at a frequency of at least 2 GHz.
In a point to point link with only 1 optical transmission channel, the combination of low chirp amplitude modulation and a high frequency SBS suppressing optical phase/frequency modulation produces an effective HFC transmitter with high optical launch power capabilities, low noise and low distortion. However, the high frequency optical phase/frequency modulation can create problems in WDM systems if it is not synchronized between transmitters. When the optical phase/frequency modulation is not synchronized, the OBI bandwidth from the transmitters beating with FWM products becomes very large. This OBI can severely degrade signal quality. Although it is possible to shift the OBI out of band by offsetting the optical transmission wavelengths from a uniform grid, the large wavelength offset requirements to shift OBI completely out of band in a WDM system with unsynchronized optical phase/frequency modulation would severely limit the number of optical channels that can be added to an OBI free WDM system. However, if the SBS suppressing optical phase/frequency modulation amplitude, frequency and phases are synchronized between transmitters, higher order harmonics of OBI cancel and the OBI RF spectrum becomes very narrow. This allows much smaller wavelength offsets and a larger number of optical channels to be added to an OBI free WDM system.
One method to synchronize the SBS suppressing optical phase modulation is to add it after the WDM mux using an optical phase modulator as in U.S. Pat. No. 7,936,997. In this case, because the same optical phase modulator modulates all the optical transmission channels simultaneously, there is inherent synchronization. However, this requires an additional optical phase modulator to be added after the optical mux, which can be costly and adds additional optical loss. If the optical sources themselves have optical phase/frequency modulation capabilities that can be used for SBS suppression, it would be desirable to use those mechanisms instead to minimize additional cost and optical loss. There may also be other reasons to add the SBS suppressing optical phase/frequency modulation to each source separately. Regardless of the reason to add the phase/frequency modulation separately to each source, there is no inherent synchronization mechanism. What is desired is means to synchronize the SBS suppressing optical phase/frequency modulation between transmitters when added before the mux in order to allow small wavelength offsets without signal degradation due to in-band OBI from FWM products beating with the transmitted signals.